The long range goal of this project is to study the oxidation of chemicals to toxic or carcinogenic metabolites by prostaglandin H synthase (PHS) and to demonstrate the importance of this enzyme system in chemical-induced toxicity or carcinogensis. We have shown that aromatic amine carcinogens, are metabolized to mutagens by PHS. PHS dependent oxidation occurred by a free radical mechanism and resulted in the formation of DNA adducts which can be used as in vivo markers for PH5-dependent oxidation. We have further studied the formation of amine mutagens by PHS using bacterial tester system that are rich or devoid of acetylase activity. Our data indicates that acetylase plays an important role in the formation of free radical mutagens. We have also studied the epoxidation of BP-7,8-diol by peroxyl radicals generated during bisulfite auto oxidation or catalysis by peroxidase. Significant enhancement of diol epoxidation occurred and significant formation of sulfite conjugates were observed. The data may explain the co-carcinogensis between bisulfite (SO2) and benzo(a)pyrene. We have also discovered a new mechanism for the formation of glutathione adducts that occurs in the absence of epoxide formation and conjugation by glutathione transferase. By this mechanism the model compound, styrene, reacts with a thiyl radical formed enzymatically by PHS. The resulting styrene- glutathione carbon-centered radical then reacts with molecular oxygen to form a peroxyl radical which eventually yields the conjugate. We have also examined the mechanism involved in the inhibition of peroxidase by methimazole. Our data indicates that methimazole is not a peroxidase inhibitor as reported, but inhibit these reactions by reducing free radical metabolites to parent compound. The results in the net inhibition of metabolism. Our data suggest that PHS is a versatile enzyme system that can catalyze a variety of reactions and appears to be important in conversion of chemicals to carcinogenic metabolites in extra hepatic tissue.